Dust Collection Downdraft Table

We need to start with a clear understanding that downdraft tables for woodworking are at best a compromise which will let much of the most dangerous fine dust escape collection. The practical problem is building a downdraft table big enough to be any use invariably fails to capture much of the chips, sawdust, and fine dust. We can engineer our downdraft table to provide the roughly 4000 feet per minute (FPM) airspeed needed to pull in our chips and larger pieces of sawdust, but even a big 5 hp motor turning a 14” or 15” diameter blower impeller will only provide that much airspeed over a round area of about 10” in diameter. This is too small an area to do much practical woodworking. With our also needing to move roughly 50 FPM outward in all directions to successfully capture most of the fine dust, we find that this same monster 5 hp blower will only collect for roughly that same 10" diametera table. We can add side boards and even a top to our downdraft table to help, but we still end up with almost all having too little airflow for good collection of either chips or fine dust. This means almost any downdraft table we build which will be larger than 10" in diameter so will be a compromise that will miss some of the chips, sawdust and fine dust.

Clearly, we cannot rely on a downdraft table to provide good long term fine dust protection as they are going to miss too much of the fine dust. This means we should always wear a good NIOSH approved dual cartridge filtered respirator whenever using a downdraft table. The dust that escapes collection will just build and build until it becomes dangerously unhealthy to work in our shops even when we are not making fine dust as almost any airflow will launch and keep this dust airborne. This means when using a downdraft table we should also be using a strong exhaust fan or big air cleaner. I also recommend building or buying a portable hood that can be connected to your dust collection system and put right next to the work as you use your downdraft table. Even with these enhancements, considerable fine dust will escape collection when we use a downdraft table. So, to stay safe whenever we use a downdraft table we also must regularly blow out our shops and clean them thoroughly with a HEPA filtered vacuum.

Even as a compromise a downdraft table will greatly reduce the amounts of dust allowed to become airborne, so I believe they are a must in most shops if we want good control over the fine dust.

A good downdraft table must provide the same things found in a good dust collection system if it is to be successful. My Dust Collection Basics page summarizes what air engineers have learned through decades of science, engineering and practical experience is required to build dust collection systems that consistently pass air quality inspections.

1. Fine dust spreads so fast that once it escapes collection there is almost no chance that exhaust fans and air cleaners will bring the amount of airborne dust down fast enough to pass an air quality inspection. We have to collect the fine dust at the source.

2. To collect the fine dust we must start with hoods and barriers around our woodworking operations to keep the fine dust from escaping before it can be captured. Most of our blades bits and cutters will launch airstreams full of dust at 100 miles an hour and faster. Even our biggest blowers used in dust collection barely move air at 60 miles an hour. Unless our hoods block or surround these fast moving air streams, control the fine dust as it is made, and deliver that dust for collection there is zero chance of passing air quality checks.

3. Lots of years of engineering work also established that we have to move enough air to pull the dust away from our tools. Airflow consists of two concerns. The airspeed defines how heavy of a particle can be moved and air volume defines the effective area we get good collection. As we know from using our shop vacuums we only collect right next to the nozzle. The reason is airspeed is what moves our dust and unlike blown air, sucked air comes from all directions so speed falls off at roughly 12 times the square of the distance. If we don’t move that nozzlehow big the area is that we can collect cover the dusty areas of our tools. and that air must move with enough speed to pull in the different sizes of dust being made. We know that most wood sawdust and chips from woodworking need about 3800 feet per minute airspeed to be sucked up into a collection hose. Most air engineers design systems with a little safety factor increasing that airspeed to 4000 FPM. both capture the dust and keep our Fine dust spreads so fast that once it escapes collection there is almost no chance that exhaust fans and air cleaners will bring the amount of airborne dust down fast enough to pass an air quality inspection. We have to collect the fine dust at the source.

4. And we must get rid of the fine dust. I live in a neighborhood and area where venting outside is not allowed. This means I had to work through what it took to provide ample filtering for my shop. Almost all dust collectors and cyclones come with fine filter bags that filter off the visible dust and freely pass the invisible dust right through. When vented inside these units quickly build up dangerously unhealthy amounts of fine dust in even the cleanest looking shops. This is why I mostly strongly recommend venting outside. Until we get to really cold temperatures a couple of IR heat dishes will keep most shops comfortable when venting outside. When you vent outside you must also provide a source of makeup air so you do not pull deadly carbon monoxide backward through your chimneys and vents.

My wife bought me a WoodStock portable plastic 24" square downdraft table from PSI. WoodStock is one of the Grizzly brands as is ShopFox. That portable downdraft table is convenient but not recommended. The inlet at 4” is too small and needs expanded to 6”. It is too large for anything less than a 3 hp dust collector if you are serious about good fine dust and chip collection. The table is poorly formed so it traps instead of directs dust. Its holes 5/16” square holes are a little too small for collecting chips and really should be a full 3/8”. Its top is a punched steel grate where the 5/16” square holes sit between 1/8” ribs. The punched metal is very sharp and will quickly destroy any project that make up its steel top grate destroy any work you put on the table because they are I often use it on top of one of my roll around tables instead of doing all the work to clean off my bigger downdraft table. It is made with a thick vacuum-formed piece plastic for the base with a 4" flex hose adapter molded in. The table is topped with a heavy steel anodized metal grill. It has 5/16" square holes separated by 3/16" ribs. That metal badly scrapes your work, so I bought an expensive non-slip perforated sanding mat that worked well until my daughter cut up my mat for a "project". Fortunately, my wife found some identical material in the form of non-slip foam cabinet liners from Wal-Mart that costs $2. Those cupboard liners look identical and work just as well as my $30 sanding pad.

To keep the table from disconnecting from the dust hose all the time and not wanting to go to the work to put on a clamp, I drilled a hole through my 6" to 4" adapter and through the 4" outlet on the table. This lets me just drop that nail in and it holds the duct in place.

This table does have its limits. It works fairly well with my PC pad sander that is known for not stirring up the dust. However, when I use more aggressive sanders this unit struggles and misses lots of dust. It really needs lift up side and back panels that would go up at least 6" to be more effective with more aggressive sanders. Using this table has also shown me that 4" duct is not enough to support a 2' square table even with my 2 hp blower! There is just not enough CFM.

Before I upgraded my shop to all 6" ducting this table worked so so at best. After upgrading, the performance of this table went from marginal to excellent. Even with the tapered transition that goes from my 6" ducting hose to the 4" outlet and no side boards as I originally planned, the performance is still very good. The air will squeeze around that small obstruction without too much loss.

I built a new downdraft table that is really a top that slips on one of the rolling tables. The design is simple. I started that table using a squirrel cage fan. I hated cleaning its 1 micron 3M furnace filters. Plus, it worked poorly with way too little airflow. I tossed the squirrel cage and filters, then went back to my 4" dust collector outlet. My first top for that table was made with half-lap spruce, all glued up pretty. It worked fine, but wore badly, especially with daughter and son sanding on it as much as on their projects. It got replaced by a piece of the heavy Masonite pegboard with my drilling out the holes from 1/4" to 3/8". I made the Masonite a little proud, put a 1/8" chamfer on the edge, and use a tapered drill for the holes with a slight countersink to keep from scratching my work (a lesson I learned the hard way). I glued on strips to make it the same height as the prior spruce top. I eventually added three 10" side skirts that can be lifted up to better trap the dust. I copied a nice design using a "J" shaped slot and a smooth bolt. The wood is simply lifted on the "J" around the bend then holds in place on the lower tip of the "J".

The way I formed the outlet and dust chute bottom is work, but sure makes a nice looking and working result. Start with a piece of 3/16" Rovel plastic (check out a local plastic store and ask for heat formable plastic). Size that sheet to fit tightly inside the downdraft table frame opening. Cut a 4" hole in the center. Secure that plastic inside the frame 1" down with a good air-tight seal. I used quarter round inside the frame placed 1" from the top, put down the plastic, caulked it in place with polyurethane sealant, then secured it tightly with another piece of quarter round on top held with brads. The brads go right through both pieces of quarter round and the plastic to make a strong joint and tight seal.

Now form the outlet. Take a piece of 6" PVC pipe and cut a slot whose width is Pi * 2 * width of the Rovel plastic. Use a band clamp to make that pipe close down tightly. This pipe will be used to form the Rovel plastic into a 6" male outlet. By using a heat gun soften around an area about 2" around that hole then press the pipe and Rovel plastic into a standard 6" PVC fitting. The result is a roughly 1 1/2" long male connector that exactly matches the pipe size needed for a tight perfect fit into our PVC pipe fittings.

Use polyurethane caulk to glue that male end into a PVC fitting. When that glue dries, insert the sizing pipe back into the hole and then carefully heat evenly that whole piece of plastic. I put a 10 pound weight on my sizing pipe to gently pull it down making for a nice smooth slope in the plastic from every direction.

Remove the sizing pipe and hook up either to a blower directly mounted below the table or to your central dust collection system. I added a pipe that goes out the side of that roll-around table that is wrapped with a spiral of wire similar to the spiral on my flex hose. This lets me screw the flex hose onto the pipe for an airtight fitting without having to mess with clamps.

I am seriously considering making one more downdraft table with my router in the center. If there ever was a tool that could use the downdraft, it would be my router.

On 1/23/04 Jim Dobbs reminded me to, "Be careful to not pull air from the top of a router. This is opposite to the direction that the router fan pushes air through the router. By pulling air from the top, you will stall the cooling air flow and burn up the router. I always put my vacuum collection in line with the cutter (edge routing) or just below the table (face routing). I learned this from experience on my first router".

1. I've just finished building your cyclone and am left with a 2 hp Grizzly 1029 dust collector. I was thinking about building the blower from that unit into a portable downdraft table. Would this work and how do I go about computing how big I can make that table? Any suggestions or advice?
   
   Some of the Grizzly 1029's had a removable 5" face cover that can be upgraded to a 6" (Depends upon year of your collector). In terms of a downdraft table I'd recommend going with the Grizzly directly mounted to your downdraft table. If you do that, you do not need to use either the 5" or 6" blower inlet mounted directly to that downdraft table. You can also go with flex hose but not quite as efficient. You really should have a 6" inlet. It all comes down to how much room you have. I made mine to roll around and at the same height as my tablesaw. By locking the rollers in place, that gives me additional room to slide stuff on and off the saw.
   
   In terms of design for the downdraft here are the steps to figure out how big you can make yours.

   1. The size of the table itself must stay small enough that it keeps up enough airspeed to pull in the fine particles and also the large particles.

   2. From my Equipment Page, Blower FAQs you can see the basic need is a minimum of about 1000 FPM to get the fine particles: Air engineers design for volume and duct speed, as the particle size gets smaller. Very Fine dusts have very little inertia and are captured well with lower velocities, but you still have to move enough volume to make sure those fine particles get drawn back into the hood. For example: gases and very light powder dust velocities would be 50-200 FPM; light powder dust at 200-1000 FPM; and powder dust at 1000-2000 FPM. Notice this is in feet per minute (FPM). Because a vacuum draws from all directions, FPM decreases at the same rate that sphere of air increases in area. From the formula for the area of a sphere at 4PIr*r we see what a huge difference a little distance makes in airflow velocity. A blower with a 6" intake that produces 800 CFM at 4000 FPM generates only 318 FPM at 1', 80 FPM at 2', and 35 FPM at 3' which is too little to take in the fine dust particles.

   3. Assuming your Grizzly will pull about 4" of resistance between the connection, resistance of the holes in the downdraft table, and filters, it should provide about 600 CFM through a 6" connection. A 6" hose has an area of 3*3*Pi = 28.27 sq. in. Divide that by 144 and we get 0.19635 sq. ft. Knowing that FPM = CFM / Duct Size in sq. ft. FPM = 600 / 0.19635 = 4074 FPM. With a target of 4000 CFM to ensure no duct plugging, the 6" pipe is near ideal for that Grizzly and most other 2 hp 12" diameter impeller blowers. If we only need 1000 FPM to move that fine dust, then we can work this backward to compute the total area for the holes in the downdraft table.

   4. From FPM = CFM / Duct size we can use a little algebra to compute Duct Size = CFM / FPM. Starting with 600 CFM from your existing blower and wanting 1000 FPM we come up with a duct size of 0.6 sq. ft., roughly three times as much area as our initial hose. Whew! With all that we come up with a downdraft table that is about a half square foot large.

   5. In fact, what we really did was compute the total area of the allowable holes in that table to ensure it gets the fine dust. If we used the thick masonite with standard 1" pegboard holes for our top, we could enlarge each hole to 1/4" (1/4" to 3/8" seems to work best). That gives each hole an area of 0.049 sq. in. per hole. The 0.6 square feet we computed as the area we had to work with turns into 0.6 * 144 = 84.6 sq. in. Divide that by 0.049 and we end up with a table that will have 86.4 / 0.04 = 1763 holes.With pegboard having a hole every 1", the square root of that, 42" would give us a fair idea of a square table size that handles small particles.

   6. At the same time, we also need to get the large particles For that we need 3000 FPM to make them move in the horizontal minimum (from the AAF tables). Divide 600 CFM / 3000 FPM and we get 0.2 sq. ft. Multiply by 144 and that gives us 28.8 sq. in. Divide that by the same hole size of 0.049 and we end up with ~ 588 holes. The square root gives us a 24" square table.

   In fact, the correct solution is going to be the smaller of the two. I made mine 2' square more by intuition than all the math, but strangely it came out pretty close. If you choke it down with 4" duct the size falls to about 16" maximum. If you use bigger holes, then the table quickly gets smaller.
   
   Make sure you provide sideboards on the table that can be raised by at least 6", preferably on all four sides. Use of J shaped holes with two wood screws makes a nice setup that will let you lower and raise those sides as convenient.
   
   With pegboard for the top you would be well off to use a little sandpaper to lightly taper the top of each hole so the holes do not hold your work (See attached Downdraft.gif) I put my hose in the center with four tapered boards made of 1/8" Melamine material glued with RooGlue, but anything would do. Leaving a lip that lets the pegboard sit just proud of the table helps lots. I leave about 1" clearance on the sides between the bottom of the pegboard and tops of the tapers with a standard 1x4 for the sides, so the taper is not a lot, but enough. Also a few blocks glued to 1/4" dowel stock makes for nice stops on the pegboard. I also use a piece of that Rubbermaid foam shelf liner that grips like a router pad, but has nice holes to let the dust through (see DowndraftPort.gif). You can mount the blower below in a roll around or go to it with hose. Also, the hole does not need to be in the center. I've designed a few with the hold on the end with the tapers going to it.